Impressive progress has recently been made in the field of optical telecommunications. Systems are not being installed that permit transmission of data at a rate of many megabits per second over distances of several kilometers between repeaters. However, since the economics of systems such as, for instance, intercontinental submarine cable systems, are strongly affected by data rate and repeater spacing, work directed towards improvement in these system parameters continues.
Optical pulses transmitted through a fiberguide suffer change and degradation due to a number of effects, even if the source spectral width is so small that the frequency spectrum of the signal is substantially transform limited, as it is assumed to be throughout the examples given in this application. Among these effects is attenuation due to absorption or scattering, i.e., a progressive loss of signal amplitude, which ultimately results in loss of detectability of the signal if the signal amplitude becomes comparable to or less than the receiver noise. Pulses are also subject to dispersion, typically resulting in a broadening of the pulse in the time domain. If such broadening is sufficiently severe, adjacent pulses can overlap, again resulting in loss of signal detectability.
Although several distinct dispersion mechanisms can be identified in optical fiberguides, for purposes of this application the significant one is chromatic dispersion. A material of index of refraction n exhibits material dispersion at a wavelength .lambda. if (d.sup.2 n/d.lambda..sup.2).noteq.0 at that wavelength. Physically this implies that the phase velocity of a plane wave traveling in such a medium varies nonlinearly with wavelength, and consequently a light pulse will broaden as it travels through such medium. In addition, signals propagating in a waveguide are subject to waveguide dispersion, which typically also is wavelength-dependent. I will refer herein to the combined material and waveguide dispersion as "chromatic" dispersion. As an example typical of magnitudes of chromatic dispersion effects, in a typical monomode fiber a 10 ps pulse of carrier wavelength 1.5 .mu.m doubles its width after about 650 m. This doubling distance is inversely proportional to the square of the pulse width, hence a 5 ps pulse will double its width after about 160 m.
If (d.sup.2 n/d.lambda..sup.2)&gt;0 throughout a certain wavelength regime then the medium is said to be normally dispersive in that regime. On the other hand, the wavelength regime throughout which (d.sup.2 n/d.lambda..sup.2)&lt;0 constitutes the anomalous dispersion regime. In silica, a regime of normal dispersion extends from short wavelengths to about 1.27 .mu.m, and an anomalous dispersion regime from about 1.27 .mu.m to longer wavelengths. Separating the two regimes is a wavelength at which (d.sup.2 n/d.lambda..sup.2)=0, i.e., at which material dispersion is zero to first order. This wavelength depends on the composition of the medium. The wavelength at which chromatic dispersion vanishes to first order similarly is composition-dependent, and in addition depends on such fiber parameters as diameter and doping profile. It can, for instance, be as high as about 1.5 .mu.m in appropriately designed monomode silica-based fibers.
A natural choice of carrier wavelength in a high data rate telecommunication system is the wavelength of first-order zero chromatic dispersion in the fiber. However, even at this wavelength, pulse spreading occurs due to higher order terms in the dispersion. See, for instance, F. P. Kapron, Electronics Letters, Vol. 13, pp. 96-97 (1977). In the case of operation at the wavelength of first-order zero chromatic dispersion, the distance over which the pulse width doubles is inversely proportional to the cube of the pulse width. In addition, the pulse distorts in a nonsymmetric way, and an oscillating tail appears. Because of the phase interference produced by the oscillating tail, a reasonable limitation of the pulse width acceptable in a system whose carrier wavelength is equal to the wavelength of first-order zero chromatic dispersion is given by 2t.sub.o L.sup.-1/3 =1.4 ps(km).sup.-1/3, in which 2t.sub.o is the pulse width, and L is the length of fiberguide over which the pulse is to be transmitted. This expression has been proposed by H. P. Unger, A. E. U., Archiv fur Electronik und Ubertragungstechnik Vol. 31, pp. 518-519 (1977). Thus, for "linear" transmission through a fiber channel of 20 kilometers, the minimum pulse width is about 3.8 ps. Hence, the theoretical maximum "linear" transmission rate over a 20 km link even at the zero group dispersion wavelength is about 0.13 Tbits/second. In reality, however, it is practically impossible to achieve and maintain exact equality between the carrier wavelength and the wavelength of minimum chromatic dispersion. Small deviations in .lambda. (.about.1 %) from this wavelength reduce the maximum bit rate to about 0.01 Tbits/second.
Recently it has been proposed to use the nonlinear change of dielectric constant (Kerr effect) of a monomode fiberguide to compensate for the effect of chromatic dispersion, i.e., to utilize solitons. A soliton is a solution of a nonlinear differential equation that propagates with a characteristic constant shape, and, for purposes of this application, I mean by "soliton" both such a solution and the corresponding pulse that maintains its shape during transmission through a fiberguide. The concept of shape-maintenance will be refined below.
When the effect on the signal pulse due to a nonlinear dependence of the index of refraction on electric field is balanced with that due to the chromatic dispersion, the possibility that the optical pulse can form a soliton has been shown to exist, and the possibility of stationary transmission of such a pulse was predicted. A. Hasegawa and F. Tappert, Applied Physics Letters, Vol. 23(3), pp. 142-144 (1973). That paper dealt with lossless monomode fibers, and, inter alia, taught the existence of a minimum pulse power, dependent on fiber parameters and wavelength, above which solitons can exist. These predictions of Hasegawa and Tappert have been verified, by demonstrating dispersionless transmission of a 7 ps pulse with a peak power of .about.1 Watt at .lambda.=1.45 .mu.m through monomode fiber for a distance of about 700 meters. See L. F. Mollenauer et al, Physical Review Letters, Vol. 45(13), pp. 1095-1098 (1980).
Utilization of the Kerr effect to achieve pulse self-confinement in multimode fibers has also been proposd recently. U.S. patent application, Ser. No. 230,322, filed Feb. 2, 1980 by A. Hasegawa, entitled "Multimode Fiber Lightwave Communication System Utilizing Pulse Self-Confinement."